Artificial sausage skins, also referred to as artificially produced sausage casings, are produced both from natural substances, that is, generally from renewable raw materials such as, for example, cellulose or collagen, and from synthetic materials.
When sausage casings made from synthetic material were introduced to the market about 40 years ago, the use of polyamide, polyester and polyvinylidene chloride mixed polymers prevailed. In the subsequent years--documented by various patent applications--improvements were made to these artificial casings above all with regard to the barrier properties and mechanical strengths. A distinct advantage in this respect is shown by coextruded sausage casings having properties specifically adjusted to the requirements of the respective sausage product being produced.
The production of sausage casings from renewable raw materials is possible only by expensive processes. Artificial sausage skins made of cellulose are produced predominantly by the "viscose process", which comprises the three steps of preparing the viscose, producing the artificial sausage skin from the viscose and finally the formation of the cellulose hydrate. As the cellulose has no melting point nor melting range, neither can it be processed thermoplastically. The process for producing such casings is very expensive and to some extent very damaging to the environment.
Thermoplastically processable synthetic materials afford the advantage of resource preservation, environmental protection with regard to waste water and waste air and distinctly decreased manufacturing costs.
In order that a synthetic material be biodegradable, the raw materials must already be biodegradable. Renewable raw materials fulfil this requirement naturally, but their properties are often inadequate for the subsequent application.
The sausage casings produced from purely renewable raw materials can however be composted.
On the other hand, many synthetic polymers are not, or are only extremely slowly, attacked by microorganisms. Predominantly synthetic polymers containing hetero atoms in the main chain are regarded as being potentially biodegradable. Among these materials, the polyesters represent an important class. Synthetic raw materials which contain only aliphatic monomers exhibit a relatively good biodegradability but their application is extremely limited owing to their material properties. Aromatic polyesters, in contrast, exhibit distinctly worsened biodegradability while having good material properties.
In recent times, various biodegradable polymers have become known (see DE 44 32 161). These possess the property of being readily thermoplastically processable and, on the other hand, of being biodegradable, that is, their entire polymer chain is split by microorganisms (bacteria and fungi) via enzymes and is completely decomposed into carbon dioxide, water and biomass. An appropriate test in natural surroundings under the action of microorganisms, such as occurs inter alia in a compost, is given in DIN 54 900. Owing to the thermoplastic behaviour, these biodegradable materials can be processed into semi-finished products such as cast films or blown films. Nevertheless, the use of these semi-finished products is very restricted. Firstly, these films display poor mechanical properties and secondly, the physical barrier properties with respect to water vapour and gases are very poor in comparison with films made of typical, but non-biodegradable synthetic materials such as polyethylene, polypropylene or polyamide.
The biodegradable materials known hitherto, owing to their range of properties, are unsuitable for use as food casings. A sausage casing made of thermoplastically processable starch is described in DE 4438961 A1. But these films exhibit poor mechanical properties as well as low resistance to water. In addition the mechanical properties are greatly influenced by moisture.
EP 0 569 146 A2 describes a polyester film consisting of a biodegradable polyester. The use of such a film as an artificial food casing is not described and neither is it obvious. EP 0 708 148 A1 likewise describes moulding compositions based on degradable aliphatic polyesters and suitable for blown films. Use as an artificial food casing is not described here either, because of the set of requirements which is not met by these materials.
The requirements placed on a sausage casing are very varied, depending on the field of application (type of sausage) and the sausage production technology used. The requirements for the following important applications are given here by way of example.
The requirements for a peeling skin arise mainly out of the industrial application and the high degree of automation associated therewith in the production of the sausages. The sausage casings, which are in principle always tightened up (110 to 150 feet tightened length), are closed on one side and the self-supporting tightened rope is pushed mechanically, without a network, onto the filling tube. The portioning (adjustment of filling weight) is carried out over the length of the sausage, that is, by cutters anchored in chains, so that a very high constancy of diameter is required. The filled sausages are subsequently introduced into a smoking chamber (temperature less than 75.degree. C.; time, depending on diameter, less than 1 hour). During the hot smoking, a skin forms as a result of protein coagulation. After the product has become red all round, it is then cooled. Afterwards, generally on the next day, the skin is automatically removed; this means that only very little adhesion of the skin to the sausage meat is required (easy peel behaviour). For these applications a diameter range of 14 to 32 mm is usual.
Raw sausages are produced from coarse to medium-fine minced beef, pork, bacon, pickling solution and spices, without the addition of water. After the sausage casings have been filled with the sausage meat mixture, which has been cooled to about -4.degree. C. and is therefore very solid, the sausages are sweated for 1 to 2 days at about 18.degree. C. to 25.degree. C. and at 98% relative humidity, in the course of which the characteristic red meat colour develops as a result of reddening all round. The sausages are then lightly smoked at 18.degree. C. to 20.degree. C. for about 3 days and subsequently matured at a maximum temperature of 18.degree. C. for several weeks in maturing rooms, with a slight movement of air. In the course of this, they lose between 25% and 45% of their weight in water. An essential prerequisite of a raw sausage casing is a high water-vapour transmission, in order that during the maturation water can diffuse out of the sausage through the casing and there evaporate at the surface. Added to this are the requirements for a high impermeability to grease and a high resistance to shrinkage with adequate strength and elasticity. These properties result primarily from the shrinkage in volume associated with the loss of water during the maturation phase, which the casing should adjust to in equal measure by corresponding shrinkage.
With the exception of sausage casings made of collagen, which are very suitable owing to the affinity of the material to the skin, it has hitherto been a problem to combine the above-mentioned properties in sausage casings made of synthetic material for use in the production of raw sausage. Besides the sausage casings made of collagen, the market has accepted sausage casings made of regenerated cellulose fibre-reinforced primarily with woven or knitted textile fibres impregnated with collagen and/or gelatine and, to a lesser extent, stitched sausage casings made of cotton fabric.